1. Field of the Invention
The invention relates to a high-pressure fuel pump with a variable delivery quantity for an internal combustion engine, having a camshaft-actuated piston that aspirates fuel from a low-pressure line into a pumping chamber and then pumps it into a high-pressure line, and having a quantity control valve connecting the pumping chamber and the low-pressure line.
2. Description of the Prior Art
In a high-pressure fuel pump of the type with which this invention is concerned, which is known from European Patent Disclosure EP 481 964 B2, the delivery quantity is regulated by providing that the quantity control valve is closed at the onset of the pumping stroke and is opened during the pumping stroke. Because of the idle volume in the pumping chamber, at the instant of opening of the outlet valve (onset of pumping in the high-pressure line and rail), the piston already has a high speed. Because of the liquid column available at this instant in the high-pressure line, which column has to be accelerated, this leads to a pressure surge. This pressure surge makes exact quantity metering in the injection of fuel into the combustion chamber more difficult and moreover causes a pulsating load on the high-pressure line and the common rail. In addition, the mechanical stresses on the high-pressure fuel pump and the camshaft, because of the surgelike load at the onset of fuel pumping into the high-pressure line, are very high.
It is the object of the invention to furnish a high-pressure fuel pump with a variable delivery quantity, in which the pressure surges in the high-pressure line and in the common rail are markedly reduced, compared to the prior art, and the mechanical stresses on the high-pressure fuel pump are reduced.
According to the invention, this object is attained by a high-pressure fuel pump with a variable delivery quantity for an internal combustion engine, having a piston actuated by a camshaft, wherein the piston aspirates fuel from a low-pressure line into a pumping chamber and then pumps it into a high-pressure line; between the pumping chamber and the low-pressure line, a quantity control valve and a separate suction valve are connected parallel, and the regulation of the delivery quantity is effected by opening the quantity control valve during the pumping stroke of the piston.
In the high-pressure fuel pump of the invention, a pressure increase takes place in the pumping chamber at the onset of the pumping stroke. As soon as the pressure force in the pumping chamber is greater than the sum of the pressure force in the high-pressure line, which force is decoupled from the pumping chamber by an outlet valve, and the spring force of the outlet valve, the high-pressure fuel pump begins to pump fuel into the high-pressure line. As soon as enough fuel has been pumped into the high-pressure line, the quantity control valve opens, so that the pressure in the pumping chamber collapses, and the outlet valve between the high-pressure line and the pumping chamber closes. Since in the above-described quantity regulation the pressure increase in the pumping chamber always takes place from bottom dead center (BDC) of the piston onward, the pressure course in the pumping chamber and hence also in the high-pressure line can be designed, independently of the rpm and the operating point of the internal combustion engine, in such a way that the pressure surges in the high-pressure line and in the common rail and the surgelike loads on the high-pressure fuel pump are reduced. The magnitude of the pressure surge depends on the speed of the cam at the instant of opening of the outlet valve.
In a variant of the invention, it is provided that each cam of the camshaft has at least a first rotational angle range, a second rotational angle range and a third rotational angle range, the bottom dead center (BDC) of the piston being located within the first rotational angle range; that after reaching BDC, in the first rotational angle range, the piston is imparted a positive acceleration by the cam; that within the second rotational angle range the stroke speed VH/omega of the piston is approximately constant; that the outlet valve of the high-pressure pump opens while the cam is passing through the second rotational angle range; and that within the third rotational angle range, the stroke speed of the piston increases until a maximum value is reached.
The second rotational angle range, with an approximately constant stroke speed VH/omega that is as low as possible, has the advantage that regardless of the delivery quantity, that is, the instant at which the outlet valve opens, depends essentially only on the rpm of the camshaft. It is thus possible, by the choice of a low stroke speed, to limit the pressure surge PS to an allowable amount, even at maximum high-pressure fuel pump rpm and maximum pressure in the high-pressure line. As a result, the injection quantity can be controlled with greater accuracy, and the aforementioned pulsating loads and surgelike loads are reduced.
In a further feature of the invention, the acceleration of the piston in the first rotational angle range, at the allowable maximum rpm of the high-pressure fuel pump, is limited essentially by the forces of inertia of the piston, so that the first rotational angle range can be kept as small as possible. This allows making the second rotational angle range correspondingly larger. Since at the onset of the pumping stroke, the piston causes only a pressure increase of the fuel in the pumping chamber and need not perform pressure increasing work counter to the pressure in the high-pressure line, the acceleration of the piston in the first rotational angle range can assume a very high value.
In a further feature of the invention, in the second rotational angle range, at the allowable maximum rpm of the high-pressure fuel pump, the piston experiences no positive acceleration or a positive acceleration that is less than the acceleration in the first rotational angle range. Compared to a constant stroke speed VH/omega, it is possible by means of a slight positive accelerationxe2x80x94on the condition that the allowable pressure surges PS in the high-pressure line are not exceededxe2x80x94to increase the stroke speed of the piston in the second rotational angle range as well and thus to attain the same pumping stroke within a smaller rotational angle range. By this provision, the maximum stroke speed of the piston can be reduced, which at high rpm of the high-pressure fuel pump leads to a reduction in flow losses at the quantity control valve upon diversion and thus enhances pump efficiency.
In a further feature of the high-pressure fuel pump of the invention, the acceleration of the piston in the third rotational angle range at the allowable maximum rpm of the high-pressure fuel pump is limited by the maximum allowable pressure, so that on the one hand the maximum piston speed in the pumping stroke is reached as quickly as possible, and on the other, no allowable stresses on the high-pressure fuel pump occur. In the third rotational angle range, the piston does have to perform work counter to the pressure in the high-pressure line.
In another feature of the invention, it is also provided that each cam has a fourth, a fifth, and a sixth rotational angle range; that the top dead center (TDC) of the piston is located between the fourth rotational angle range and the fifth rotational angle range; that the positive acceleration of the piston by the cam becomes negative in the fourth rotational angle range; that in the fifth rotational angle range, the piston is imparted a negative acceleration by the cam; and that within the sixth rotational angle range, the stroke speed of the piston is negative and approximately constant. As a result, the intake stroke is made possible with reduced mechanical stress on the fuel pump and less cavitation. This advantage is still greater if in the fourth and fifth rotational angle range, the change in speed of the piston is approximately constant.
In one embodiment of the high-pressure fuel pump, the quantity control valve is a magnet valve that is open when without current, so that impermissible pressures in the fuel feed pump are prevented even if the quantity control valve or its triggering fails.
In a further feature of the invention, at the transition from the sixth rotational angle range to the first rotational angle range, the intake speed decreases slowly, so that the overflow losses from excessively late closure of the inlet valve are reduced.